1. Field of the Invention
This invention relates to heat-resistant thermoplastic copolymers, and more specifically to injection moldable polyamide imide resins excellent in both heat stability and melt viscosity characteristics at a temperature of from about 300.degree. C. to about 400.degree. C.
2. Description of the Prior Art
It is well known that an aromatic polyamide imide, excellent in heat resistance, is obtained by the polycondensation of an aromatic tricarboxylic acid anhydride or derivative thereof and an aromatic diamine or derivative thereof. However, the aromatic polyamide imides which have been previously proposed are not necessarily satisfactory for use as melt molding materials because of lack of total balance of thermal stability at melt molding, melt viscosity at melt molding, and the mechanical characteristics of the melt molded articles.
For example, U.S. Pat. No. 3,920,612 discloses a polyamide imide polymerized from trimellitic acid chloride anhydride and 4,4'-diaminodiphenyl ether as shown by the formula ##STR1## which is excellent in heat resistance but in practice, cannot be melt molded because the fluid starting point and the thermal decomposition point of the polyamide imide are too close.
Polyamide imides polymerized from trimellitic acid anhydride and 4,4'-diaminodiphenylemthane or m-phenylenediamine as shown by the formulae ##STR2## as in, for example, U.S. Pat. No. 4,016,140 have thermal decomposition points of less than 300.degree. C., and melt molding of them is considerably difficult because their heat resistance is not satisfactory, and still further, the fluid starting points and the thermal decomposition points are too close. Even if they could be molded, the molded articles obtained would have very poor mechanical characteristics.
Japanese Kokai 74-129,799 discloss a polyamide imide polymerized from trimellitic acid chloride anhydride and 4,4'-sulfonylbis-(p-phenyleneoxy)dianiline as shown by the formula ##STR3## This polyamide is excellent in melt molding because the difference between the fluid starting point and the thermal decomposition point is more than 50.degree. C. and the thermal stability and melt viscosity at melt molding are also excellent. However, the flexibility of the diamine unit is so large that the properties of the molded articles, particularly the heat distortion temperature, do not attain satisfactory levels.
In order to eliminate the drawbacks of the known polyamide imides, copolymerization of trimellitic acid chloride anhydride and two different kinds of diamines has been proposed in, for example U.S. Pat. No. 3,748,304. But, the combinations of two kinds of diamines as shown in this patent are of five different systems, as follows: ##STR4##
Polyamide imides obtained from systems containing ##STR5## are of inferior thermal stability and show a decline in thermal stability in proportion to the content of these diamines, so that these polyamide imides are not satisfactory as melt molding materials. Furthermore, the system containing ##STR6## poses a big obstacle to efficiency of production of the polymer because of the low polymerization activity of ##STR7##
In any event, the copolymers obtained utilizing the above systems are not altogether satisfactory in practice.
Therefore, it is a primary object of the present invention to provide a melt moldable aromatic polyamide imide which is excellent in heat resistance, mechanical characteristics and electrical characteristics.